Just two brief observations.
First, if we didn’t have a death penalty, the Left would be arguing that life imprisonment and other lengthy sentences are violations of the 8th Amendment. You cannot be certain in the assumption that we could do better without the death penalty. At the moment, the Left has to understate its argument against imprisonment because having people serving long sentences is important to its argument that we don’t need the death penalty. Once the death penalty was gone, though, they would be arguing for release of incorrigible, violent felons — and some of them, inevitably, would be released.
Second, some of the most violent crimes, and acts of war, including the 9/11 attacks and the 1993 World Trade Center bombing, have been ordered, plotted, and — in the case of the al-Qaeda terrorist who maimed a prison guard in 2000 — carried out in prison by terrorists who were not executed for their prior crimes.